1. Field of the Invention
The present invention relates to a technology for embedding an electronic watermark in image data.
2. Description of the Related Art
Along with development of information networks, it has become easy to obtain electronic information created by other people. Accordingly, problems such as tampering with copyrighted electronic information without notice to an author have arisen. In order to prevent or restrict the tampering, an electronic watermark technology is calling attention.
There are many proposals for technologies for detecting tampering using an electronic watermark into image data. For example, Japanese Patent Application Laid-open No. 2003-204429 discloses a technique of detecting tampering of an image in which an electronic watermark is embedded, irrespective of the image being compressed or non-compressed.
According to the above technique, an electronic watermark is embedded into all areas of image data to detect tampering. However, it is not always necessary to detect tampering in all areas of the image data. In other words, if an electronic watermark is embedded into all areas, even when a change is made in a part of the image data that has no problem with being changed, the image data is determined as “tampered”.
For example, in image data indicating a price for advertisement, a change in a picture in the image data does not cause a problem as long as a change in text in the image data indicating the price is prevented. This is because even when the picture is changed, the image data is still effective in advertising as long as the price is displayed correctly. In this case, transmission and reception of the image data through network is considered to have no practical problem. However, when displayed amount of the price is changed, it should be detected as tampering and be treated properly.
In this example, even when the picture is changed, the image data is still effective in advertising as long as the price is correct. Therefore, even when the picture in the image data is changed, right holders of the image data may wish to spread the image data through the network, rather than detecting the image data as “tampered” to limit spread of the image data. If the image data is detected as “tampered”, transmission and reception of the image data can be limited. This stops extensive transmission of the image data, even though a change made is allowable, thereby hindering an advertising effect.